A new unified constitutive model was developed to predict the two-stage creep-aging (TSCA) behavior of Al-Zn-Mg-Cu alloys. The particular bimodal precipitation feature was analyzed and modeled by considering the primary micro-variables evolution at different temperatures and their interaction. The dislocation density was incorporated into the model to capture the effect of creep deformation on precipitation. Quantitative transmission electron microscopy and experimental data obtained from a previous study were used to calibrate the model. Subsequently, the developed constitutive model was implemented in the finite element (FE) software ABAQUS via the user subroutines for TSCA process simulation and the springback prediction of an integral panel. A TSCA test was performed. The result shows that the maximum radius deviation between the formed plate and the simulation results is less than 0.4 mm, thus validating the effectiveness of the developed constitutive model and FE model.